In the past, a semiconductor device has been utilized not only for control of an electromagnetic signal in a computer, but also for power control of a power source in industrial equipment such as electrical cars of railroads, electric vehicles, machine tools, refrigerating machines, etc. Since the semiconductor device which is utilized for such power control is designed for control of power, it generates a large amount of heat. A board on which such a semiconductor device is mounted is required to have a high heat dissipating efficiency.
As a material of a heat dissipating plate on which a semiconductor device for power control is mounted and which dissipates heat generated from the semiconductor device, in the past, high thermal conductivity copper and copper alloy have been utilized.
However, copper and a copper alloy have larger coefficient of thermal expansion compared with the silicon, gallium arsenide, etc. forming the semiconductor device. Accordingly, generation of heat by the semiconductor device sometimes caused cracks to form due to the difference in coefficient of thermal expansion between the semiconductor device and the heat dissipating plate. The formation of such cracks as a result sometimes becomes a factor causing the deterioration of heat dissipating property of the semiconductor device and destruction of the semiconductor device.
For this reason, when reliability of the semiconductor device is particularly necessary, from the viewpoint of preventing the occurrence of cracks due to the difference in coefficient of thermal expansion between the semiconductor device and the heat dissipating plate, molybdenum, tungsten, or their alloys have been utilized for the materials for the heat dissipating plate.
However, these materials of heat dissipating plates have the defect of low thermal conductivities. Increasing the volume of the heat dissipating plate to make up for the defect of the low thermal conductivities leads to an increase in mass due in part to the large densities of these materials.
Therefore, to improve the ease of handling and the running performance in transport equipment such as electric cars of railroads and electric vehicles, a heat dissipating plate of a semiconductor device for power control use which realizes a high thermal conductivity and a lower coefficient of thermal expansion and which is light in weight has been desired.
To meet these requirements for a heat dissipating plate, Patent Document 1 and Patent Document 2 propose a composite material of copper and diamond, and a composite material of copper and copper (I) oxide respectively.
However, although the composite material which is disclosed in Patent Document 1 achieves both a thermal conductivity over 600 W/(m·K) and a coefficient of thermal expansion under 5×10−6, it has the defect that it is high in cost and further is inferior in cuttability when adjusting the dimensions etc because it utilizes diamond.
On the other hand, the composite material which is disclosed in Patent Document 2 achieves both a thermal conductivity over 200 W/(m·K) and a coefficient of thermal expansion under 16×10−6. In addition it is easy to produce and realize at a low cost. However, since the densities of copper and copper (I) oxide are respectively 8.9 kg/dm3 and 6.4 kg/dm3, the requirements for lighter weight is not satisfied.
Therefore, as art relating to the composite material satisfying such requirements for lighter weight, Patent Documents 3 to 7 all propose composite materials which contain aluminum alloy and silicon carbide. Since the composite materials described in Patent Documents 3 to 7 have densities of aluminum alloy and silicon carbide of respectively around 2.7 kg/dm3 and around 3.2 kg/dm3, they satisfy the requirements for lighter weight.
The art disclosed in Patent Document 3 is a method of production of a composite material by so-called impregnation. Specifically, this is the method of molding particles or fibers of silicon carbide to form aggregates of particles or fibers of silicon carbide, that is, porous preforms, bringing them into contact with molten aluminum, and impregnating the voids in the porous preforms with the molten aluminum alloy.
Further, the art disclosed in Patent Document 4 is a method of production of a composite material by so-called casting. Specifically, it is a method of mixing molten aluminum alloy and particles of silicon carbide, then casting them.
Further, the arts disclosed in Patent Document 5, Patent Document 6, and Patent Document 7 are methods of production of composite materials of aluminum alloy and silicon carbide by so-called powder metallurgy. Specifically, they are methods of mixing aluminum powder or powder of aluminum alloy and particles of silicon carbide, then sintering them.